The present invention relates generally to a method for producing a magnetic recording medium. More particularly, the present invention relates to a coating method and a coating apparatus for simultaneously coating a plurality of different organic-solvent-based non-magnetic coating liquids or magnetic coating liquids on a strip formed of a continuously moving non-magnetic substrate to form a multi-layered structure on the substrate by successively laminating the coating liquids.
In recent years, the requirement for recording of various information on a magnetic recording medium having a reduced thickness at a higher density has increased. In view of this recent trend, a single magnetic layer coated on a strip of non-magnetic supporting material is being replaced by a multi-layered structure. This is because a magnetic recording medium having a magnetic layer formed in a multi-layered structure provides substantially improved magnetic recording properties in the form of an increase of a magnetic data storing capacity, compared with a magnetic recording medium having a magnetic layer formed in a single layer. In practice, the multi-layered structure is required to exhibit a sectional shape consisting of two to several layers.
A method for producing a magnetic recording medium having a multi-layered structure by repeatedly forming a coated layer and then drying the same one by one is disclosed, e.g., in Japanese Patent Publications Nos. 54-43362 and 58-43816 and Japanese Patent Laid-Open Publications Nos. 51-119204, 52-51908 and 53-16604.
With the foregoing conventional method, however, since the steps of coating and drying are repeatedly performed, the method is accompanied by low productivity, and, moreover, requires a large and expensive apparatus to implement. In addition, according to the conventional method, the magnetic recording elements are not arranged in correct alignment with each other along an interior boundary plane between adjacent coated layers, causing undesirable tape modulation noise or the like to be readily generated. Moreover, the adjacent layers do not sufficiently adhere to each other. The foregoing disadvantages arise from repeatedly performing the same forming step the same number of times as the number of layers to be coated, which itself results in low productivity.
On the other hand, a method for coating a coating liquid to form a multi-layered structure is employed when photosensitive materials, heat sensitive papers or the like are produced. In this connection, a phenomenon of transforming a sol of gelatin into a gel of the same by adjusting the temperature of the coating liquid is utilized for producing photosensitive materials. In this case, the viscosity of the coating liquid can easily be determined corresponding to the sol state most suitably employed during each coating operation as well as the preferred gel state during each drying operation. In the case where a sheet of paper is used as a supporting substrate, moisture in the coated layer is absorbed by the paper no matter how much the viscosity of the coating liquid is reduced during each coating operation. Thus, the coating liquid becomes more viscous after completion of the coating operation, such that unwanted disturbances of the coated layer in the course of subsequent coating operations is reliably prevented.
If a newtonian fluid is used as the coating liquid for heat sensitive materials and photosensitive materials as mentioned above, its physical properties largely vary depending on the static viscosity (which can easily be determined by the composition of the coating liquid). Moreover, the viscosity of the coating liquid can be comparatively easily adjusted in the above-described manner. Thus, plural coating liquids can simultaneously be coated on the supporting substrate to form a multi-layered structure.
However, a magnetic coating liquid serving as a magnetic recording medium is a non-newtonian fluid, its viscosity varying coating operations depending on various conditions. This makes it very difficult to properly control the physical properties of the magnetic coating liquid attributable to the viscosity of the liquid. Thus, whether or not the aforementioned problem can be overcome when the viscosity of the coating liquid is adjusted depending upon the operative conditions is greatly dependent upon the skill and experience of the operator. In the circumstances as mentioned above, it is very difficult to assure that each coating operation can be achieved at a high productivity while maintaining a more stable quality than in the conventional coating method.
In view of the circumstances as mentioned above, a method for forming a coated layer with a multi-layered structure with only a single coating step or with continuous coating steps has already been proposed and employed in practical use. However, with respect to the coating method disclosed, e.g., in Japanese Patent Laid-Open Publication No. 62-124631 as a method for simultaneously coating a plurality of different coating liquids to form a plurality of magnetic layers with a multi-layered structure, when an organic-solvent-based non-magnetic coating liquid and a magnetic coating liquid or two kinds of magnetic coating liquids are simultaneously coated to form a multi-layered structure, color fluctuation and longitudinally extending stripes are liable to occur when the coating operations are performed at a low speed, compared with the conventional method wherein coating and drying steps are sequentially repeated. Thus, there are problems with the formation of a multi-layered structure according to this method in that the quality of the coated product is remarkably degraded with respect to the electromagnetic conversion properties and appearance, and a coating liquid on the upper layer side cannot be uniformly coated on the lower coated layer, resulting in the two layers failing to be properly formed, depending on the procedure of preparing the coating liquids. Moreover, the coating liquid forming the upper layer cannot completely be coated on a coated layer serving as a lower layer. These problems worsen as the quantity of each coating liquid is reduced (i.e., coating is achieved to form a thinner layer).
In addition, an apparatus for simultaneously coating a plurality of different coating liquids to form a plurality of layers is disclosed in, e.g., Japanese Patent Laid-Open Publications Nos. 58-109162 and 63-88080.
With the coating apparatus disclosed in Japanese Patent Laid-Open Publication No. 63-88080, the liquid disturbance between adjacent layers can be reduced, and the degree of freedom in selecting coating liquids prior to simultaneous coating of the coating liquids to form a plurality of layers can be increased, even though physical properties of one coating liquid differ considerably from those of another coating liquid. However, due to the trend of recording information on a magnetic recording material at ever-increasing densities, there arises a problem that as coating liquids are coated on a magnetic recording material to form three or more layers, a very thin coated layer, especially, an upper coated layer having a thickness of, e.g., about 1.0 .mu.m or less cannot be stably formed because of a phenomenon whereby coated liquid is spattered away from the lower coated layer.
In view of the foregoing problem, a method for forming a recording layer serving as a magnetic recording medium having a very small thickness wherein a dummy layer is used as a means for increasing the thickness of a coating liquid applied during each coating operation and for reducing the thickness of a product of a coated recording layer has been developed. Specifically, according to the developed method, a dummy layer, i.e., a non-magnetic layer is used for the lower layer of the magnetic recording layer, and coating liquids are simultaneously coated to form a plurality of layers in such a manner as to allow the magnetic recording layer to be placed on the dummy layer.
However, the dummy layer and the non-dummy layer have a thixotropic nature, and, moreover, a difference of physical properties specific to the thixotropic nature appears between both the layers, causing physical properties (apparent viscosity or the like) specific to these coating liquids to differ largely from each other under substantially the same conditions. Thus, it has been found that the behavior of each coating liquid varies depending on the liquid merging speed in the liquid merging region where the non-dummy layer is coated on the dummy layer so that undesirable states such as stripe-shaped coating, irregular coating, spattering of the coating liquid away from the lower layer or the like are liable to appear.
According to the approach disclosed in Japanese Patent Laid-Open Publication No. 58-109162 and illustrated in FIG. 6 therein, a coating apparatus for forming three coated layers is illustrated as a multi-layer coating apparatus. However, since the disclosed coating apparatus includes doctor edge planes each of which has a long length (corresponding to a length as measured in the direction of travel of a supporting substrate), it is presumable that the physical properties of each coating liquid vary due to a shearing force imparted to the coating liquid when the doctor edge plane has an excessively long length, resulting in the behavior of the coating liquid becoming unstable. In addition, it is known that each doctor edge plane has an optimum length specific to the coating apparatus which varies depending on the kind of coating liquid and coating conditions. Especially as each coating liquid is increasingly coated on the supporting substrate with a reduced thickness, it is required that the length of the doctor edge plane as measured in the direction of travel of the supporting substrate be reduced.
However, as described in Japanese Patent Laid-Open Publication No. 58-109152, when it is intended that the length of the doctor edge plane be reduced while respective slits extend in parallel with each other, an intermediate block interposed between adjacent slits (i.e., an intermediate block defining the slits) must be made thin, causing it to be readily flexed by an exterior force such as hydraulic pressure or the like exerted on it during a coating operation. Thus, there arises a problem that irregular coating is liable to be performed.
To solve the problem of irregular coating, the coating apparatus employs a mechanism for reducing the length of the doctor edge and increasing the mechanical strength of the intermediate block by inclining each slit in such a manner that the slits are collectively arranged at the forwardmost end of a coating head.
However, as the inclination angle of each slit increases, a coating liquid merging angle for causing the coated layer to extend in the direction of travel of the supporting substrate increases correspondingly. Thus, there arises a problem that the behavior of the coating liquid in the coating liquid merging region varies largely, resulting in the failure of achieving a thin coating layer.
In addition to the aforementioned merging angle, the coating apparatus is significantly affected by operative conditions in which each coating liquid has thixotropic physical properties and each coating layer is formed with a very small thickness. Thus, with the conventional coating method wherein a coating head is constructed with a merging angle determined based on the operator's experience, it is very difficult to form a plurality of coating layers each forming a thin layer on a mass production line.
Moreover, in the case where an orienting treatment is conducted for the magnetic layer after completion of a coating operation, when a coating liquid is coated on the supporting substrate with a very small thickness, a solvent contained therein is vaporized a very short time before the magnetic layer is dried. Thus, the solvent is dried to some extent before the coated magnetic layer is subjected to the orienting treatment, making it difficult to perform the orienting treatment, and when the orienting treatment is excessively progressively conducted, the magnetic properties of the magnetic recording medium become adversely affected in the form of deterioration of a magnetic orienting property of the magnetic recording medium.
As is apparent from the above description, in the case where coating liquids are coated on the supporting substrate to form multi-layers each having a very small thickness, the problems of color tone fluctuations across each coated layer due to deflection of the intermediate block in the coating head and the scattering of the coating liquid away from the coated layer are left unsolved. In addition, the problem whereby the orienting property of the magnetic recording medium is deteriorated even though the coating liquid is coated on the supporting substrate while forming multi-layers to some extent is left unsolved. For this reason, when a magnetic tape produced by practicing the conventional coating method under a condition where e.g., a magnetic dispersant is employed as a coating liquid is used as, e.g., a video tape, there arise problems that noise associated with video properties of the magnetic tape is generated, and, moreover, the image resolution of the magnetic tape is degraded.